Preformable, homogeneous epoxy resin compositions which are not self-reactive at ordinary or "pre-preg" process temperatures, but which become self-curing when more strongly heated, are known.
However, none of the three prior art systems of the preceding type known to the present inventor provided organo-soluble curing agents in which the latent reactive function is a phenolic hydroxyl group. Rather, NH, NCO or metal salt complexes with imidazoles are provided. It will be seen from the following summary descriptions that at least two of the known systems also have other shortcomings as latent epoxy curing agents.
U.S. Pat. No. 4,111,917 discloses the use of certain, thermally decomposable epoxy-miscible urethanes as latent amines. The urethanes are formed by reacting isocyanates with alcohols which tend to be readily dehydrated to olefins. When heated to epoxy curing temperatures, these urethanes decompose to form primary amines, CO.sub.2 and the dehydrated alcohol. One end of the urethane molecule may be attached to an epoxy resin by a thermally stable urethane group derived from a secondary alcoholic hydroxyl in the epoxy but the other end must terminate in a urethane group which does decompose as above described. Thus, the utility of the patented system is limited to thin film applications, in which degassing is practical.
U.S. Pat. No. 4,448,816 discloses the use of cyclic ureas, in which one of the nitrogen-attached hydrogens is replaced with a --CO--C.sub.n H.sub.2n --NCO group, as "one-can", latent isocyanate curing agents. The isocyanate group is reacted with an active hydrogen in a polyol or polymer to form a stable adduct group and--upon heating--each cyclic urea moiety in the adduct rearranges to a linear OCN--C.sub.n H.sub.2n --NH-- group. The hydrogens in the resulting --NHCO-- moieties are presumably oxirane reactive but the patented system would appear to be impractical as a latent curing agent in one part epoxy compositions.
U.S. Pat. No. 4,487,914 discloses the use, as latent curing agents, of epoxy-miscible complexes of metal salts with 2:1 or 1:1 adducts of monoepoxides (such as phenyl glycidyl ether) with imidazoles. To the best of the present inventor's knowledge, no oxirane-reactive compounds which contain masked phenolic hydroxyls and will form organo-soluble complexes with metal salts have been disclosed.